Computer Audio Interface:  You’ll need one of these because you can’t plug your instruments and microphones straight into your DAW.  It’s basically a box connected to your DAW into which you plug your instruments, and mixing desk if you have one.   In simple terms, instruments and microphones generate analogue data, which must go into your PC and be converted to digital data on the way in (by an 'AD converter') as computers only deal in digital data.  You can then use software to manipulate this data as you desire.  When you play music back, so you can hear it on your headphones and/or speakers, the digital data is being converted back to analogue (by a, yes, 'DA converter').   Why is this important?  (It is!)  See 'Latency' below.           

As with your computer, do some research based on what your setup will be mainly used for.  I do all my own recording, overdubbing as necessary, so I don’t need a lot of simultaneous inputs for other musicians and I have a mixing desk anyway if I wanted to do that.  So the interface I use is the RME HDSPe Aio which is in PCI Express Card format so goes inside my PC.  Pictured left are three examples of interfaces that connect to your computer via USB, by Native Instruments, Focusrite and Presonus.  There are lots of sites on the internet that compare various interfaces, so researching one that matches your needs is fairly straight forward.       

In my setup two cables (L & R for stereo) go to from my mixer to the back of the Aio card in my PC, and two similar cables come back the other way.  So, I can record in stereo into my desk and therefore into my PC and playback recordings in the opposite direction.

Computer MIDI Interface:  If you’re going to be working with MIDI, which most of us do as it’s a great tool, you’ll need one of these because the Delta 66 above carries only audio signals.  MIDI signals are different so you’ll need a similar interface but one which carries MIDI information between your DAW software and any MIDI instruments that you have such as synthesisers and drum machines.  I have the M-Audio Midisport 4x4 (see photo left) which enables me to have a synthesiser and two drum machines connected simultaneously to my PC so I can pass MIDI information between them and my DAW - both ways.  MIDI interfaces are simple and cheap as MIDI data is simple and small, unlike audio interfaces which have to 

Mixing desk - do I need one?  (that's mine on the left)  No, recording software packages come with a desk or console view which emulates a studio mixing desk’s controls if that’s how you’d like to work. It’s a matter of personal preference; there’s a lot to be said for the tactile satisfaction of being able to press real buttons and slide real channel sliders, which explains why even in these days of ultra hi-tech digital studios you still see mixing desks.  But if you want to record your band, for example, and need to plug in a number of different instruments and microphones a desk can be very handy.  Having said that, there are computer audio interfaces on the market with multiple inputs which will cope with that.

If you have ambitions to expand your home project studio and record for profit, then later on down that road you might wish to acquire a desk ('mixer' or 'console') as clients will probably expect to see one!  Again, there is plenty of information and comparisons on the internet to peruse, but make sure you research mixers best suited to a home studio; there are also mixers for live mixing at venues which can have slightly different specifications.   

Latency:   When recording a vocal or an instrument, latency is slight time delay that you hear between (1) the live performance of the musician or vocalist being recorded and (2) this recording then playing back from the PC and through monitors or headphones.  The delay between what is played and what is heard back happens because the signal that enters the audio sound card doesn’t exit the sound card at the same time: it has to make a 'round trip' from the real (analogue world) into AD converters, into the PC, out through DA converters and back out to drive headphones or speakers.   This is very fast, but if it isn't fast enough you hear 'doubling'.  It can cause major problems for musicians, engineers, and producers in the studio when recording a performance or take, as the time difference can make it impossible for anything to be played in time with the rest of the track.  There’s no way to completely get rid of this latency, but there are things you can do to minimize it.   A latency time of around 10ms or less usually means that it won’t affect the recording process. Above 10 ms, the effect starts to become noticeable.  In fact, there is latency in the real world.  If a guitarist stands 20 feet or more in front his amplifier whilst playing on a large stage, there is more latency than if he/she were standing 2 feet in front of the amplifier.  In this case, the difference is probably too small to be significent. 

There are things you can do to minimise latency, the main one being to make sure you invest in a good computer with a powerful chip, plus a high-spec audio interface.  Generally, the more you pay, the higher quality AD/DA converters, so invest as much as you can to avoid frustrations further down the line.  There is an excellent, more detailed description of latency and ways to combat it at this web site.